In the field of conservation there is a continuous challenge to develop new and sustainable treatments, which have no or a minimal impact on the microstructure of the object. In the conservation of photographs, removal of corrosion products from the surface is a delicate issue; conservators/restorers should choose the most suitable method. Non-thermal remote atmospheric plasma cleaning might be a possible alternative to currently used methodologies. The advantages presented by this method are: no presence in the interaction volume of highly reactive species, such as ions (the main cause of the non-selective physical etching of the surface by low-pressure plasmas); no mechanical contact with the fragile surface and so reduction of the risk of damage during treatment; eco-sustainability compared to conventional conservation-restoration techniques based on chemical solutions.
However, it is still unclear how the chemical etching of the afterglow affects the aged photographs. In the paper “Reclaiming the image of daguerreotypes: Characterization of the corroded surface before and after atmospheric plasma treatment” by Eva Grieten et al. published in Journal of Cultural Heritage, Volume 28, 2017, Pages 56-64, ISSN 1296-2074, https://doi.org/10.1016/j.culher.2017.05.008, the authors present an in-depth study of the changes induced by this method, by a multi-analytical approach using optical microscopy, scanning electron microscopy, different types of transmission electron microscopy and X-ray absorption fine structure, and focusing on daguerreotypes. The H2-He afterglow removes S from an Ag2S or Cu2S layer which results in a nano-layer of metallic Ag or Cu on top of the deteriorated microstructure. In case the corrosion layer is composed of Cu-Ag-S compounds, the proposed setup can be used to partially remove the corrosion. These alterations of the corrosion result in an improvement in the readability of the photographic image.
This research was developed during the EU project PANNA 'Plasma And Nano for New Age "soft" conservation' (ENV‐NMP.2011.2.2‐5, Grant agreement No. 282998) to which ICMATE (as IENI at the time) took part in the materials characterization and new solutions development from a chemical point of view.